This invention relates to composite materials, and particularly, to composite materials which exhibit magnetic properties.
The coercive force in referring to magnetic materials is the reverse magnetic field which must be applied to a magnetic material in order to make the magnetization of the material vanish. Correspondingly, the coercivity in referring to a material is that coercive force of a magnetic material in a hysteresis loop, wherein the maximum magnetization approximates the saturation magnetization. Providing materials having controlled coercivities are desirable for fabricated articles such as transformers, (where minimum coercivity is required), magnetic tapes (where a controlled intermediate coercivity is required), and magnets (where a maximum coercivity is required). For example, by altering the size, shape and composition of coercive materials, numerous coercive properties can be obtained.
Materials having coercive properties have a particle size above a certain size at which superparamagnetization occurs. Maximum coercive properties are observed by particles having a single domain stage. Composites prepared from coercive particles can exhibit less coercivity. Mixtures of coercive particles and superparamagnetic particles have been disclosed in E. Kneller, Magnetism and Metallurgy, Vol. I (A. E. Berkowitz and E. Kneller, ed.) Academic Press (1969). However, such mixtures are disclosed as being present at very low concentrations as relatively dilute mixtures. Thus, such a disclosure provides no direction towards the preparation of composites which are useful in numerous applications.
It would be highly desirable to provide a useful composite having magnetic properties which composite exhibits coercive properties which can be effected by superparamagnetic materials.